Conventionally, a control system for an internal combustion engine in which a variable intake mechanism changes the amount of intake air drawn into cylinders as desired has been proposed in Patent Literature 1. This control system controls the amount of intake air (intake air amount) by the variable intake mechanism, and is comprised of an air flow sensor for detecting the flow rate of air flowing through an intake passage of the engine, a crank angle sensor for detecting a state of rotation of a crankshaft, an accelerator pedal opening sensor for detecting an opening of an accelerator pedal (hereinafter referred to as “the accelerator pedal opening”), and a controller to which are inputted detection signals from these sensors. The controller calculates an engine speed based on the detection signal from the crank angle sensor, and the intake air amount based on the detection signal from the air flow sensor. Further, the engine is provided with a throttle valve mechanism and a variable valve lift mechanism as variable intake mechanisms. The throttle valve mechanism changes the flow rate of air flowing through the intake passage as desired, and the variable valve lift mechanism changes the lift of each intake valve (hereinafter referred to as “the valve lift”) as desired.
As will be described hereinafter, in the control system, the intake air amount is controlled by the controller. First, it is determined based on the engine speed, the accelerator pedal opening, and the intake air amount, in what operating load region the engine is. Then, when it is determined that the engine is in a low-engine speed and low-load region including an idling region, the valve lift is controlled to a predetermined low lift by the variable valve lift mechanism, and the opening of the throttle valve is controlled to a value corresponding to the engine speed and the accelerator pedal opening by the throttle valve mechanism. On the other hand, when it is determined that the engine is between a medium-engine speed and medium-load region and a high-engine speed and high-load region, the throttle valve is controlled to a fully-open state, and the valve lift is controlled to a value corresponding to the engine speed and the accelerator pedal opening.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. 2003-254100
In the above conventional control system, it is sometimes impossible to properly calculate the intake air amount, due to a low resolution of the air flow sensor. For example, some type of internal combustion engine has an intake passage whose diameter is set to a large value (i.e. which is configured to have a large diameter) so as to reduce flow resistance within the intake passage to enhance the charging efficiency of intake air in cylinders. When the conventional control system is applied to the engine configured as above, the flow velocity of intake air assumes a very low value when the engine is in a low-engine speed and low-load region, and hence the conventional control system cannot properly calculate the intake air amount due to the low resolution of the air flow sensor, which degrades the accuracy of the intake air amount control. As a result, when the amount of fuel to be supplied to a combustion chamber is controlled based on the intake air amount thus calculated, there is a fear that the resulting degraded control accuracy lowers fuel economy and increases exhaust emissions. Furthermore, since the engine to which the conventional control system is applied is provided with the throttle valve mechanism, this causes an increase in the flow resistance within the intake passage.
Further, in a high-load region of the engine, the reliability of the intake air amount calculated based on the detection signal from the air flow sensor is sometimes degraded, due to occurrence of intake pulsation or an excessively high flow velocity of the intake air. In such a case as well, the above described problems are brought about.
On the other hand, in the ignition control of the engine, a method is conventionally employed which uses an engine speed and an intake air amount as load parameters indicative of load on the engine, and ignition timing maps having map values of ignition timing set in advance in association with the load parameters. In the above-described engine with the intake air passage having a large diameter as well, it is envisaged that ignition timing is controlled by the above method. However, as described above, in the conventional control system, the intake air amount cannot be properly calculated in the low-load region of the engine due to the low resolution of the air flow sensor. This degrades the accuracy of the ignition timing control. Further, the charging efficiency of intake air in the cylinders varies with the engine speed, causing the maximum value of the intake air amount in the high-load region where knocking starts to occur to also vary with the engine speed. Therefore, in the high-load region of the engine, it is necessary to set the number of set values of the intake air amount and the maximum values of the same in a fine-grained manner while making them different from each other. This causes an increase in the number of data of the ignition timing maps to increase the number of times of measurement for sampling the data as well as the capacity of a storage medium, such as a ROM, resulting in increased manufacturing costs. Moreover, the accuracy of the ignition timing control is degraded also in the engine in which the reliability of the intake air amount calculated based on the detection signal from the air flow sensor lowers in the high-load region of the engine, as described above.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a control system for an internal combustion engine, which is capable of enhancing the accuracy of fuel control and ignition timing control even when there is a possibility that the reliability of a calculated intake air amount lowers, and enables reduction of manufacturing costs.